Certain vehicles are propelled at least part of the time using electrical energy drawn from a high-voltage DC battery pack. The battery pack energizes one or more polyphase electric traction motors via a power inverter. Hybrid electric vehicles selectively use an internal combustion engine as a source of input torque to a transmission, alone or in conjunction with the traction motor(s). Extended-range electric vehicles use a smaller engine only when needed, and solely to power an electric generator. Battery electric vehicles forego use of a gas engine altogether, and instead operate using stored electrical energy or regenerative braking energy. All three vehicle configurations can operate at least part of the time solely via electricity in what is referred to as an electric vehicle (EV) mode.
In all of the above vehicle embodiments, the high-voltage DC battery pack is used to alternatively store and deliver the substantial amounts of electrical energy needed for driving the fraction motor(s). The battery pack may consist of multiple battery modules each containing multiple cylindrical or flat/tabular battery cells. Effectively dissipating heat generated by the battery cells in operation is essential to optimizing overall vehicle performance. As a result, battery thermal systems are used in conjunction with such battery packs to circulate a volume of a suitable heat transfer fluid through the battery pack and associated power electronics. The same thermal system can also be used to heat the battery pack as needed.